There are a number of different methods of conducting handoff in a cellular system. Handoff is the act of transferring a call with a subscriber (e.g., mobile, portable, etc.) from one transceiver, a source transceiver, to a second transceiver, a target transceiver. In frequency division multiple access (FDMA) systems when a call is handed from one transceiver to another, the connection to the first transceiver is broken and quickly established with a second transceiver. While handoffs can be performed quickly, the quality suffers during handoff and connections can be missed; resulting in calls being dropped.
To resolve these types of problems, the ideas of soft and softer handoffs were developed. A soft handoff is a handoff between one site and another. A softer handoff is a handoff between one sector and another sector, both located at the same site. The basic concept of soft/softer handoff is that the connection is established with the second transceiver before the connection with the first transceiver is broken.
The implementation of the present invention will be described in conjunction with direct sequence code division multiple access (DS-CDMA) systems, but is not limited to those applications. In DS-CDMA systems, the transmissions of a plurality of signals are carried over the same spectral resource. The typical spectral resource is a wide band (1.2 MHz) system. However, there are also narrow band (9.6 KHz and 200 KHz) systems that are being implemented. The transmitted signals are spread throughout the band, using a known code. This same code is then used by the receiver to despread the signal.
The limitation on the number of signals which can be carried in the band is closely associated with the total power of the signals in the band. A more detailed description of power control in DS-CDMA systems is provided in U.S. patent applications: "Method for Compensating for Capacity Overload in a Spread Spectrum Communication System" having Ser. No. 07/783,751 filed on Oct. 28, 1991; and "A Method for Controlling Transmission Power in a Communication System" having Ser. No. 07/907,072 filed on Jul. 1, 1992, both assigned to the present assignee.
While the soft/softer handoff techniques provide a higher quality signal to the receiver, it is at the expense of overall system capacity. When multiple transmitters are transmitting the same signal, excess power is being put into the system. This excess power reduces the number of subscribers which the system can accommodate.
This problem has been illustrated in current test systems in which it has been shown that 50% to 60% of the subscribers in the system are in handoff mode. From a system standpoint, one-third or more of the system transmit power is not being utilized by the subscribers. Instead, this excess power is only adding to the noise.